The tropical Andes in southern Ecuador constitute a hotspot of plant (especially trees and bryophytes) and animal (especially birds, bats, arctiid and geometrid mothes) diversity. However, data on small animals such as testate amoebae as an important component of the soil and aboveground community are lacking. Variations in density, diversity and community structure of testate amoebae along altitudinal transects in tropical regions are largely unknown. Testate amoebae colonize almost any habitat but are most abundant and diverse in soils with high humidity, high organic content and slow rates of decomposition. They preferentially feed on certain bacteria thereby affecting the taxonomic composition and metabolic activity of microbial communities. By altering microbial activity testate amoebae affect nutrient cycling in particular in ecosystems where earthworm populations are depleted. The present thesis investigates the density and diversity of testate amoebae in litter, soil and aboveground habitats along an elevational gradient of tropical mountain rain forests in southern Ecuador, evaluates correlations with biotic and abiotic factors, and proves the role of nutrient limitation. The first part of the thesis evaluates if the diversity and density of testate amoebae in the studied tropical mountain rain forests (1) changes with altitude, (2) is as high as in respective forests of the temperate and boreal zone, and (3) is typical for tropical southern hemisphere locations. Species composition of testate amoebae is investigated at three altitudes (1000, 2000 and 3000 m) and two horizons (L and F/H/Ah). A total of 135 species and intraspecific taxa of testate amoebae were found. Rarefaction plots suggest that only few more species are to be expected. The results suggest that species richness of testate amoebae does not decrease continuously with elevation; rather, it peaks at intermediate levels. Further, the data suggest that diversity, but not density of testate amoebae in tropical forests exceeds that in temperate forests. The great majority of testate amoebae species of the studied tropical mountain rain forests are geographically widespread, including temperate regions; however nine of the species (6.7%) are considered tropical, some of these species likely represent Gondwana relicts. The second part of the thesis analyses altitudinal changes in microbial biomass and community composition and seeks relationships between microorganisms and testate amoebae. Microbial biomass, fungal biomass and microbial community structure at three altitudes (1000, 2000 and 3000 m) and in two soil layers [L/F layer (Layer I) and underlying H/Ah layer (Layer II)] was investigated. Basal respiration, microbial biomass and concentration of ergosterol generally declined from Layer I to Layer II and peaked at 2000 m. Compared to temperate forest ecosystems microbial biomass and ergosterol concentrations were generally low. Presumably, low microbial biomass in soils of tropical forest ecosystems is due to high temperatures associated with high respiration but also low litter quality, with the latter declining with altitude. These conclusions are supported by the fact that at higher altitudes the microbial community changed from a bacterial-dominated to a fungal-dominated system. Parallel to microbial biomass and ergosterol concentrations the density of testate amoebae peaked at 2000 m. However, compared to microbial parameters changes in testate amoebae communities between the two layers were less pronounced. The data suggest that density and community structure of testate amoebae are driven by the availability of food resources (bacteria and fungi) which at high altitude decrease with increasing moisture and decreasing pH. The third part of this thesis investigates if testate amoebae in soils of tropical montane rain forests are bottom-up regulated. To prove this hypothesis, carbon (C) and nutrients (N, P) were added to the soil and the response of microorganisms and testate amoebae to the additional resource supply was investigated. We assumed that microorganisms will benefit from the additional resources and that this will propagate to the next trophic level, i.e. protists represented by testate amoebae, since microorganisms are the major food resources for testate amoebae. The results indicate that saprotrophic fungi in tropical montane rain forests are mainly limited by carbon whereas gram positive and gram negative bacteria benefit from increased availability of P. Testate amoebae suffered from increased dominance of saprotrophic fungi in glucose treatments but benefited from increased supply of N presumably by increased availability of high quality detritus, certain bacteria and increased performance of endosymbiotic algae. The results show that testate amoebae of tropical montane rain forests are controlled by bottom-up forces relying on specific food resources rather than the amount of bacterial biomass with saprotrophic fungi functioning as major antagonists. Microbial food webs in soil therefore may be much more complex than previously assumed with trophic links being rather specific and antagonistic interactions potentially overriding trophic interactions. The fourth study of this thesis evaluates if testate amoebae from aboveground epiphytic communities respond to altitudinal gradients (macroscale) and to the height the epiphyte is located on the tree (microscale). Changes in diversity and density of testate amoebae in epiphytic microhabitats along tree height from 0 to 1 to 2 m and regional macrohabitats (forests at 1000, 2000 and 3000 m) were investigated. A total of 115 taxa of testate amoebae were found. The results suggest that species diversity and density peak at intermediate elevation. Furthermore, density in aboveground habitats was higher than that in litter and soil (weight based data). Density and diversity of testate amoebae significantly changed along both the micro- and macroscale, however, variations in density were more pronounced at the macroscale whereas variations in diversity were more pronounced at the microscale, suggesting that microhabitat characteristics are most decisive in selecting for testate amoebae species. Overall, the results document that with a total of 166 species determined in this study testate amoebae in Ecuador are highly divers compared with other tropical and temperate forests. Density and diversity of testate amoebae are controlled by bottom-up forces with their density depending on the availability of high quality detritus resources, certain bacterial groups, the performance of endosymbiotic algae and antagonistic interactions with saprotrophic fungi. The availability of food resources decreases with increasing elevation, with increasing moisture and with decreasing pH, resulting in maximum density and diversity at intermediate elevation.